Federal Trade Commission Definition for
Spandex Fiber: A manufactured fiber
in which the fiber forming substance is a long-chain synthetic
polymer comprised of at least 85% of a segmented polyurethane.
(Complete FTC Fiber Rules
here.)

Basic Principles of Spandex Fiber Production
 The polymer chain is a segmented
block copolymer containing long, randomly coiled, liquid, soft
segments that move to a more linear, lower entropy, structure.
The hard segments act as virtual cross-links that
tie all the polymer chains together into an infinite network.
This network prevents the polymer chains from slipping past each
other and taking on a permanent set or draw. When the stretching
force is removed, the linear, low entropy, soft segments move
back to the preferred randomly coiled, higher entropy state,
causing the fiber to recover to its original shape and length.
This segmented block copolymer is formed in a multi-step proprietary
process. It is extruded into a fiber as a monofilament threadline
or for most products into a multiplicity of fine filaments that
are coalesced shortly after they are formed into a single threadline.

Spandex Fiber Characteristics

Can be stretched repeatedly and still recover
to very near its original length and shape

Generally, can be stretched more than 500%
without breaking

Stronger, more durable and higher retractive
force than rubber

Lightweight, soft, smooth, supple

In garments, provides a combination of comfort
and fit, prevents bagging and sagging